This invention relates to filtration methods and apparatus and, more particularly, to filtration thickening methods and apparatus which can be used, for example, in ore processing and tailing thickening operations in the mining industry.
In mining operations, water, with and without chemical additives, is commonly used to separate the ore from the finely crushed rock and earth particles. All matter that is not ore is known as mine tailing and it is by far the largest portion of material involved in the operations. Mine tailing has no commercial value and is disposed of with a considerable amount of process water in large man-built tailing ponds provided for this purpose. In order to reduce the cost of recycling the process water and also to reduce the size and hence the cost of the tailing ponds and tailing conveyance equipment it is common practice to remove as much process water as possible before conveying the waste solids to the disposal pond. A currently used method for this purpose is to pass the tailing through settlement basins known as thickeners which may be as large as 450 feet in diameter. As the solid particles settle by gravity on the bottom of the basin, a rake pushes the particles toward an outlet at the center of the basin. The mixture of collected solids together with the process water that escapes through this outlet is known as tailing underflow. The underflow is pumped from the center outlet to the tailing pond. Clear water flows over the top of the wall around the perimeter of the basin, and is recovered as process water for recycling. The cost of building and operating basin thickeners can be very high. Furthermore, such gravity sedimentation-type basin thickeners can generally thicken the tailing to only 10 to 50 percent solids by weight, whereas it would be desirable to achieve values of 30 to 75 percent solids, respectively, the value being dependent on the type of tailing. Therefore a considerable amount of water still remains with the tailing. The use of chemical flocculants in such machinery is used to increase the rate of sedimentation but it has little effect on the final concentration of solids in the underflow.
In the known filtration thickening apparatus, the suspension of solids that is to be thickened is introduced to one side of a filter cloth. The perforations of the filter cloth are sized to allow the passage of the liquid component of the suspension, but to prevent the passage of the solids component. Most filter-cloths consist of a woven fabric, commonly made of synthetic fibers, either loosely or tightly woven, as determined by the grain size of the suspended solids and by the desired clarity of the resulting filtrate.
A known way of forcing liquid through the filter-cloth is to raise the pressure of the suspension to a higher value than the pressure of the filtrate on the opposite side of the filter cloth. Another known way of forcing liquid through the filter cloth is by applying a suction or vacuum on the filtrate side of the filter cloth. As the liquid flows through the filter-cloth, the suspended solids impinge on it, developing thereon a layer of solids known as filter cake. As the filter cake thickness grows, the passage of liquid through the filter-cloth is impeded. In order to clear the filter cloth at specific intervals of time or at specific pressure differentials that are eventually reached between the suspension and filtrate sides, the pressure differential on the two sides of the filter cloth is reversed. This causes a reversal in the flow of liquid and therefore some of the filtrate is forced to return to the suspension side. In so doing, the filtrate disengages the accumulated filter cake which then settles in the form of sheets and strips through the suspension compartment to the bottom of the equipment from where the thickened suspension can be withdrawn. This is commonly termed the backwash cycle.
Because the filter cloth is not normally strong enough to withstand the desirable filtration pressure, nor in some cases the desired backwash pressure, it is common practice to provide support means on either one or both sides of the filter cloth. These support means generally consist of rigid cages, screens, or perforated plates. The supports allow the passage of liquid but prevent the filter cloth from bursting.
Suspensions from different industrial processes vary chemically, and in both size and shape of the particles that form their solids component. It can therefore be reasonably expected that various filter cakes will differ in their thicknesses, weights, toughnesses and the facility with which they disengage from the filter cloth. A particular filter cake may be very soft and light, such as results from filter thickening of natural clay slimes, a waste product from the processing of phosphate ore. On the other hand, the filter cake may be relatively dry, heavy and tough such as the product of filter thickening of tailing waste from base metal mining operations. It has been found that the gradation and specific gravity of the solids play a primary role in the ease or difficulty with which filter cake can be disengaged from the filter cloth. Generally, the coarser the fraction, the easier the build up of filter cake on the filter cloth and thus the easier the disengagement of the filter cake from the filter cloth. To be commercially practical, the filter or thickener apparatus must be capable of operating effectively and efficiently with a soft and light filter cake, or with a coarse filter cake, the apparatus being designed as required to suit one or several of the various industrial processes.
One major drawback of known filter thickeners is that a very large quantity of backwash filtrate or cleansing liquid is required to disengage the filter cake from the cloth and to clean the filter cloth to permit the cycle to be repeated. The more backwash liquid that is used, the less efficient becomes the system, the efficiency being measured by the net gain of filtrate over the filtrate lost during the backwashing cycle. It has been proposed to utilize continuous processes having multiple tubular filter elements which operate sequentially. However, these continuous processes do not obviate the problem of large losses of fluid during the backwash cycle of any individual filter element.
It is therefore, an object of the present invention to provide a method and apparatus for efficient filtration and thickening of tailing to reduce the cost of disposal.
It is a further object of this invention to provide a method and apparatus for removing liquid from a slurry or solids suspension at a high flow rate while being relatively inexpensive to construct and operate.
It is yet another object of the present invention to provide a method and apparatus for drawing water from tailing at a high flow rate that requires a relatively small area of space for the apparatus, is simple to construct and operate, and has a minimum of moving parts.
It is still another object of the present invention to provide several filtering methods and apparatuses designed for the various types of filter cake encountered in filtration.
Filtration thickening apparatus according to a preferred embodiment of the present invention includes a thickener tank to which the tailing or other liquid-solid suspension is supplied. The tank has a receiver portion at the bottom for the accumulation of filter cake and a filtrate compartment at the top for collection of filtrate. Individual tubular filter units are suspended in the tank below the filtrate compartment and above the filter cake receiver portion. An overflow pipe is spaced above the filter units in the filtrate compartment for draining the filtrate liquid from the tank. A supply pipe enters the tank above the filter cake receiver portion. The filter units preferably include either an accumulator grating or a filter cloth which is sealed along narrow bands to form a pattern of filtering areas or both. A centrifugal pump supplies the tailing from a holding tank to the thickener tank and a valve controls the discharge of filter cake from the receiver portion of the thickener tank.
The process of filtration thickening a liquid-solids suspension in accordance with a preferred embodiment of the present invention includes pumping the suspension into a thickener tank. The liquid portion of the suspension is forced to flow through the filter cloth as filtrate and the solids portion collects on the filter cloth as filter cake. The filter cloth preferably is separated into filtering areas by liquid impermeable strips to improve the sloughing off of the filter cake during backwashing. A portion of the filtrate is intermittently backwashed to slough off the solids portion from the filter cloth. A centrifugal pump supplies the tailing to the thickener tank, forcing the liquid through the filter units and upwardly to a filtrate compartment from which the clarified liquid is discharged by gravity. Backwashing is accomplished by stopping the pump and simultaneously opening the underflow valve to discharge a portion of the filter cake underflow which has accumulated in the receiver portion of the tank. As soon as the pump is stopped the hydrostatic head of the suspension in the thickener tank causes part of the suspension to drain back through the pump and into a holding tank which is disposed at a lower elevation. This action, together with the discharge of some filter cake underflow, creates a lower head in the thickener tank suspension than in the clarified liquid filtrate compartment, thus filtrate flow is reversed. Reversed flow from the filtrate compartment causes the clarified liquid to flow through the filter units in the reverse direction, thereby dislodging the filter cake and causing it to fall into the receiver at the bottom of the tank from whence it will be removed by the opening of the underflow valve in subsequent backwash cycles. The process can be repeated at time intervals as selected according to the rate at which the filter cake builds up on the filter units.